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This blog is about a Google Summer of Code project that aims to improve EclipseFP, the Haskell plugin for Eclipse. Read more about the blog, the author and the project in a series of three introductional posts.

More robust Scion client code

July 19, 2009 — Thomas ten Cate

New post, new theme! With this wider theme, the screenshots will no longer fall off the edge.

Although I did not plan to “build one to throw away”, this largely became reality nonetheless. By now I’ve done so many refactorings on the Scion client code that hardly a single line of the original remains. But I’m finally happy with the result.

The Scion client previously used its own command queue, which ensured that commands were sent to the server one after another. After some investigation, I replaced that by Eclipse’s job scheduling mechanism. With proper scheduling rules, we can keep the scheduler from running two commands simultaneously, so it is as powerful as the home-brew command queue. If one command depends on the result of another, we can make the second job fall asleep and have the first wake the second up when it’s done. And it has several advantages over the old system:

We can now set priorities on commands, so that interactive commands (such as computing the text of a tool tip) take precedence over longer-running commands (such as builds).

Commands will automatically be shown in the progress report windows.

The sender of a command can either run it asynchronously, or wait for the command to complete before continuing.

Of course, it’s simply less code to maintain and have bugs in!

Secondly, I introduced a Scion “manager” class that ensures that there is always one Scion server running per project. If the server crashes, it is restarted automatically and brought back to the same state as its previous incarnation.

Thirdly, all significant errors are now logged to the Error Log window in Eclipse. There should no longer be a need to turn on error tracing to troubleshoot problems in the Scion client/server system.

Finally, if the server fails to start in the first place, this is probably because its executable program could not be found. Most likely this is because the plugin is used for the first time. Therefore, we pop up a helpful dialog:

Little things like this go a long way, making it easier to get up to speed with EclipseFP. In the same vein, I will start working on a “new project from existing source” feature that is well known from Java development in Eclipse. (EclipseFP is already able to import Cabal packages from a .tar.gz, but strangely not from a directory on disk!)